This invention relates to thermoplastic polymers, such as polyolefins or other difficult to extrude polymers, having improved extrusion characteristics. In another aspect it relates to the use of a dendritic material to improve the extrusion characteristics of such thermoplastic polymers, particularly in combination with other process additives. In a still further aspect it relates to the use of a dendritic material in a thermoplastic polymer further comprising an antiblock agent.
Westover, R. F., xe2x80x9cMelt Extrusionxe2x80x9d, Encyclopedia of Polymer Science and Technology, Vol. 8, John Wiley and Sons, (1968) pp 573-581 states that for any polymer there is a certain critical shear rate above which the surface of the extrudate becomes rough and below which the extrudate will be smooth. He further states that in order to achieve the highest possible flow rate from the extruder and to achieve the most uniform extrudate cross section the processor must control extrudate roughness or distortion. Some of the various types of extrudate roughness and distortion observed in high and low density polyethylene are described in Rudin, A., Worm, A. T., Blacklock, J. E., xe2x80x9cFluorocarbon Elastomer Aids Polyolefin Extrusion,xe2x80x9d Plastics Engineering, March 1986, pp. 63-66. Rudin et al. state that for a given set of processing conditions and die geometry, a critical shear stress exists above which polyolefins like linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene suffer from melt defects. At low shear rates, defects may take the form of xe2x80x9csharkskinxe2x80x9d, a loss of surface gloss, which in more serious manifestations, appears as ridges running more or less transverse to the extrusion direction. At higher shear rate the extrudate can undergo xe2x80x9ccontinuous melt fracturexe2x80x9d becoming grossly distorted. At rates lower than those at which continuous melt fracture is first observed, LLDPE and HDPE can also suffer from xe2x80x9ccyclic melt fracturexe2x80x9d, in which the extrudate surface varies from smooth to rough. The authors state that lowering the shear stress by adjusting the processing conditions or changing the die can avoid these defects to a certain extent, but not without creating additional problems. For example, extrusion at a higher temperature can result in weaker bubble walls in tubular film extrusion, and a wider die gap can affect film orientation. The authors state that the use of fluorocarbons elastomer processing aids can permit the operation of extruders with narrower die gaps and lower melt temperatures. Others have also described the use of fluorocarbons elastomers as processing aids, see for example, De Smedt, C., Nam, S., xe2x80x9cThe Processing Benefits of Fluoroelastomer Application in LLDPE,xe2x80x9d Plastics and Rubber Processing and Applications, 8, No. 1, (1987), pp. 11-16; U.S. Pat. Nos. 3,125,547 (Blatz), 4,581,406 (Hedberg et al), and 4,855,360 (Ducchesne and Johnson).
The use of highly branched macromolecules as an additive in a thermoplastic composition has been described. These macromolecules are often referred to as dendrimers, dendritic polymers, arborescent polymers or hyperbranched polymers. For the purpose of this application, these materials will be referred to as dendritic materials. For example, PCT Patent No. WO 97/19987 discloses the use of a dendrimer comprising functional end groups into which an additive has been introduced. By proper selection of end groups, a dendrimer may be designed to carry an additive for delivery into a thermoplastic with a minimum of compatibility or bleeding problems. The use of a dendrimer to improve or eliminate sharkskin in LLDPE tubular film at low shear rates was reported by Hong, Y.; Coombs, S. J., et al., Polymer, 41(2000), 7705-7713.
Thermoplastic polymers used in extrusion processes typically contain a variety of additives in addition to processing aids, such as hindered amine light stabilizers, slip agents, fillers and antiblocking agents. A number of these commonly used additives have been shown to interfere with fluoropolymer processing additive performance. See WO 95/21887. The interference caused by antiblocks has been lessened with the addition of polyethylene glycol: B. V. Johnson, T. J. Blong, J. M. Kunde, D. Duchesne, TAPPI 88 Polymers, Laminations, and Coating Conference, 249-256 (1988) and B. V. Johnson, J. M. Kunde, SPE ANTEC Tech. Papers, 46, 1425, (1988).
A need still exists for additive systems which are designed to improve extrusion characteristics that function within a broader range of shear rates. A need also exists for alternate systems to improve extrusion characteristics of compositions comprising antiblock agents.
One aspect of the present invention provides an extrudable composition comprising
(A) A major amount of a non-fluorinated thermoplastic host polymer, and a minor amount of a mixture of
(B) fluorocarbon polymer, and
(C) dendritic material.
Preferably, the dendritic material and the fluorocarbon polymer are present in such relative proportions and at concentrations which in combination or in concert, are sufficient to reduce extrusion pressure and/or melt defects, i.e. those defects, such as sharkskin, continuous melt fracture and cyclic melt fracture. A major amount in this case means 50 wt % or greater and a minor amount is less than 50 wt %.
Generally, the ratio of the weight of the fluorocarbon polymer to the dendritic material in the extrudable composition is 10 parts fluorocarbon polymer to at least I part dendritic material. Preferably, the weight ratio is in a range of 10:1 to 1:10. Where the extrudable composition is a final extrudate or final product, for example, a film, the concentration in the extrudable composition of the fluorocarbon polymer is at least 0.005 weight percent and that of the dendritic material is at least 0.0005 weight percent. This weight percent is based on the total weight of the extrudate. The preferred concentration ranges are 0.005 to 0.2 wt % fluoropolymer and 0.0005 to 2 wt % dendritic material.
In another aspect, this invention provides a polymer processing aid composition comprising fluorocarbon polymer and dendritic material such that the ratio of the weight of the fluorocarbon polymer to the dendritic material in the processing aid composition is less than 10:1. Preferably, the weight ratio is in a range of 10:1 to 1:10. Optionally, the processing aid composition further comprises other components or adjuvants, e.g. antioxidants, antiblocking agents, pigments and fillers, normally added to thermoplastic polymers. The concentration of the fluorocarbon polymer, dendritic material, and other adjuvants in said processing aid composition can vary depending upon the processor""s requirements, but generally, the fluorocarbon polymer and dendritic material will be the major or predominant component of the processing aid composition. If the processing aid composition is provided in the form of a masterbatch, the carrier or host polymer of the masterbatch will be a large component of the processing aid composition.
In still another aspect, the advantage of the synergistic combination of fluoropolymer and dendritic material is shown in its best light when the host polymer of the extrudable composition comprises an additional component, such as an antiblock agent, that is known to lessen the beneficial effect of polymer process additives.
In a further aspect, this invention provides a method for reducing pressure encountered during the extrusion process, e.g., gate pressure, comprising:
a) providing a fluorocarbon polymer,
b) providing a dendritic material,
c) combining the fluorocarbon polymer and the dendritic material with a host polymer to form an extrudable composition, and
d) extruding the composition,
wherein the fluorocarbon polymer comprises at least 0.005 wt. % and the dendritic material comprises at least 0.0005 wt. % of the extrudable composition.
The pressure reduction realized by this process allows for higher throughput, for example by increasing the extruder screw speed to bring the pressure back to the starting value, a higher volume of material is extruded in a given amount of time. Alternatively, if the process is operated at the lower resulting pressure, less energy will be consumed. Both results are beneficial and some combination of the two may be desirable for specific applications.
In a still further aspect, this invention provides a method for reducing the presence of surface or melt defects encountered during the extrusion process, e.g., melt fracture, comprising:
a) providing a fluorocarbon polymer,
b) providing a dendritic material,
c) combining the fluorocarbon polymer and the dendritic material with a host polymer to form an extrudable composition, and
d) extruding the composition,
wherein the fluorocarbon polymer comprises at least 0.005 wt. % and the dendritic material comprises at least 0.0005 wt. % of the extrudable composition.